The present invention relates to the field of radio frequency reflector antenna systems and more particularly to such antenna systems where the antenna feed blocks a portion of the radiation from the antenna reflector.
Many reflector antennas suffer from blockage. Blockage is a particular problem in antenna systems in which a reflecting dish is illuminated by a feed located along the axis of the dish. In such an antenna system a portion of the radiation emitted by the feed and reflected from the reflector strikes the feed and is blocked by the feed and excluded from the reflected collimated beam produced by the reflector. This exclusion of radiation from the collimated beam is referred to as blockage. Blockage can adversely effect the antenna's beam pattern by reducing the gain of the antenna, reducing its directivity, and increasing the amplitude of the beam pattern sidelobes.
The effects of blockage can be reduced by shrinking the size of the feed. However, that is not always feasible and there is always a limit to how much a feed may be shrunk. Another alternative is to displace the feed from the axis of the dish to avoid blocking the central portion of the beam. Unfortunately, such an offset feed for a dish reflector causes undesirable cross polarization. Thus, an offset feed introduces its own detrimental effects. As a consequence, offset feeds are not a favored solution to the blockage problem.
An improved reflector antenna system design is needed which reduces the adverse effects of blockage without itself introducing other undesired effects.